The invention relates to a vehicle brake system comprising a brake assembly providing a service brake and a parking brake having a brake disc assembly in common. The invention also relates to a working machine, a method for controlling a vehicle brake system, a computer program, a computer readable medium, and a controller for a vehicle brake system.
A working machine, e.g. a wheel loader or an articulated hauler, is usually provided with a bucket, container or other type of implement for digging, carrying and/or transporting a load.
For example, a wheel loader has a lift arm unit for raising and lowering an implement, such as a bucket. The lift arm comprises hydraulic cylinders for movement of a load arm and the implement attached to the load arm. Usually a pair of hydraulic cylinders is arranged for raising the load arm and a further hydraulic cylinder is arranged for tilting the implement relative to the load arm.
In addition, the working machine is often articulated frame-steered and has a pair of hydraulic cylinders for turning/steering the working machine by pivoting a from section and a rear section of the working machine relative to each other.
The hydraulic system generally further comprises at least one hydraulic pump, which is arranged to supply hydraulic power, i.e. hydraulic flow and hydraulic pressure, to the hydraulic cylinders. The hydraulic pump is driven by a power source, such as an internal combustion engine or an electric motor.
In a traditional driveline for a working machine, an internal combustion engine, usually a diesel engine, is provided, as well as a transmission, drive shafts, and wheel axle sets with hubs. However, alternative working machine drivelines may be considered, e.g. electric hybrid drivelines, with parallel or series hybrid configurations, or purely electric drivelines. A series electric hybrid driveline can comprise an internal combustion engine, a generator, an electric energy storage arrangement, power electronics, as well as electric motors and transmissions in the wheel hubs.
A typical working machine is also provided with service brakes as well as parking brakes. The service brakes are commonly of a wet multi-disc type with hydraulic application. For safety reasons, parking brakes are often arranged so as to be applied by a spring force and released with pressure acting against the spring force.
In a working machine with a traditional driveline, the parking brake is often mounted on or inside the driveline transmission, acting on the transmission output axle or the driveshaft. Thereby, one single parking brake assembly can act on all wheels of the working machine. However, this is not possible on a series electric hybrid driveline or an electric driveline with electric hub motors. Instead, parking brake assemblies need to be integrated in the wheel hubs of the working machine. Furthermore, due to the lack of space in the hub, it is advantageous to integrate the parking brake and service brake assemblies, by allowing the parking brake and the service brake to share disc assemblies.
Where service brake and parking brake functions share brake assemblies, if the service brake is applied while the parking brake is applied as well, there could be a risk that the brake assembly becomes overloaded due to the combined forces from the service brake circuit and the parking brake circuit. This could result is damages to the brake assemblies. For example the friction material of the brake discs can be overloaded, which in turn may damage the friction material.
A known solution to solve said problem with combined service brake and the parking brake forces is known as anti-compounding. This involves using the pressure of the service brake application to counteract the spring force of the parking brake, thereby reducing the parking brake force while the service brake force is applied. An example can be found in EP 1826085A2, suggesting a double check valve assembly which allows, if a spring activated parking brake is applied, for service brake pressure to be delivered to reduce the force that the spring exerts on the brake while the service brake function is applied.
A problem with known anti-compounding solutions rests in the fact that they are intimately linked with the design of the brake assembly. For example, the effective piston areas in the brake assembly, for the service brake actuation and the parking brake release, respectively, have to be mutually adapted so that a service brake application is balanced with a suitable compensation of the parking, brake release. This might complicate the design of the brake assembly, for example by providing a requirement that counteracts other requirements on the brake assembly.
It is desirable to avoid excessive brake forces in brakes with service as well as parking brake capacities, while providing flexibility to the design of the brakes.
A vehicle brake system according to an aspect of the invention is provided comprising
a brake assembly providing a service brake and a parking brake having a brake disc assembly in common,
a service brake circuit,
a parking brake circuit, and
a brake control arrangement adapted to control the service brake by selectively providing a fluid communication between a pressurising arrangement and the service brake via the service brake circuit, the brake control arrangement further being adapted to control the parking brake by selectively providing a fluid communication between the pressurising arrangement and the parking brake via the parking brake circuit, and by selectively draining the parking brake circuit,
wherein the vehicle brake system comprises a draining circuit adapted to drain, the service brake circuit to avoid overload of the brake disc assembly.
The working fluid of the vehicle brake system may be e.g. hydraulic fluid or air. It is understood that each circuit in the vehicle brake system comprises one or more conduits for control of respective functions in the system. It is also understood that the pressurising arrangement comprises suitable parts for pressurising the working fluid, e.g. a pump, and possibly a pressure accumulator. In addition, it is understood that the brake disc assembly can be any suitable type of brake disc assembly, e.g. including a wet brake or a dry brake.
The invention provides for the draining circuit to drain the service brake circuit. This could be done for example by releasing pressure in the service brake circuit to a pressure dumping device. Depending on the type of working fluid in the vehicle brake system, such a pressure dumping device could be a hydraulic return tank or to an air exhaust device.
The draining of the service brake circuit to avoid overload of the brake disc assembly differs from known anti-compounding solutions. More specifically, when applied to a brake system with a spring load activated and fluid pressure released parking brake, the invention provides for an attempt to activate the service brake, when the parking brake is engaged, to not change the braking force exerted by the spring of the parking brake. Instead any service brake pressure increase, resulting from the service brake activation attempt, may be diverted to a return tank or released.
Thus, as opposed to known anti-compounding solutions, the invention does not require the effective piston areas in the brake assembly to be mutually adapted, so that a service brake application is balanced with a suitable compensation of a parking brake release. This considerably simplifies the design of the brake assembly. Thus, with the invention excessive brake forces, in brakes with service as well as parking brake capacities, may be avoided, while flexibility to the design of the brakes is provided.
Preferably, the draining circuit is adapted to drain the service brake circuit while the parking brake circuit is drained. As understood from the reasoning above, where the parking brake is spring load activated and fluid pressure released, draining the service brake circuit while the parking brake circuit is drained, means that loads on the brake disc assembly from the service brake circuit cannot be added to the spring load of the parking brake.
As understood, for the spring load activation, the parking brake may comprise an elastic element adapted to provide or exert a force for the application of the parking brake. The elastic element could be a spring, e.g. a steel spring or a gas spring. The parking brake may present a parking brake piston and be adapted to be released by fluid pressure on the piston counteracting the three of the elastic element.
Preferably, the draining circuit is adapted to drain the service brake circuit via the parking brake circuit. Thereby, the draining circuit may comprise a non-return valve adapted to allow a fluid flow from the service brake circuit to the parking brake circuit, and to prevent a fluid flow from the parking brake circuit to the service brake circuit. In such embodiments, where the parking brake is spring, load activated and fluid pressure released, the pressure in the parking brake circuit when the parking brake is released, will prevent flow from the service brake circuit to the parking brake circuit via the non-return valve, and thereby the service brake may be applied upon suitable control of it. Also, the non-return valve will prevent any flow from the pressurised parking brake circuit to the service brake circuit, when the latter is not applied. However, upon engagement of the parking brake, with a coinciding drainage of the parking brake circuit, the draining circuit, with the non-return valve, being adapted to drain the service brake circuit via the parking brake circuit, will provide for automatically drainage of the service brake circuit as well, thereby avoiding the risk of overloading the brake disc assembly by service brake application in addition to the parking brake application. Thus, the overload protection may be provided only with passive components, such as the non-return valve, and thereby a genuinely robust and reliable solution is provided. I.e. the solution for parking brake application and brake overload protection does not rely on any electric valves or software control, but will instead be operational automatically, e.g. upon a brake system power failure or a control system failure.
Preferably, where the brake control arrangement comprises a service brake control device adapted to selectively provide the fluid communication between the pressurising arrangement and the service brake via the service brake circuit, the parking brake circuit is adapted to provide the fluid communication between the parking brake and the pressurising arrangement independently of the service brake control device. Thereby, the parking brake may be controlled independently, and no manipulation of the service brake device is necessary in order to control the parking brake, for example to release the latter. The brake control arrangement comprises a parking brake control arrangement adapted to selectively provide the fluid communication between the pressurising arrangement and the parking brake via the parking brake circuit.
A working machine according to an aspect of the invention is provided comprising a vehicle brake system according to any embodiment herein. The working machine may be in the form a wheel loader or an articulated hauler. The invention is particularly beneficial where the working machine is provided with a plurality of wheels, and, at each wheel, an electric propulsion motor for transferring torque to the respective wheel. The reason for the electric motors at the wheels might be that the working machine comprises a series electric hybrid propulsion system or an electric propulsion system. As also suggested above, due to the lack of space in the hub, the parking brake and service brake assemblies may be integrated by allowing the parking brake and the service brake to share disc assemblies. The draining circuit for draining the service brake circuit to avoid overloading the brake disc assembly at parking brake release will thereby be advantageous.
A method for controlling a vehicle brake system according to an aspect of the invention is provided comprising a brake assembly providing a service brake and a parking brake having a brake disc assembly in common, a service brake circuit connectable to the service brake, and a parking brake circuit connectable to the parking brake, the method comprising
controlling the vehicle brake system so as to assume a first state in which a fluid communication is provided between the parking brake and a pressurising arrangement, for keeping the parking brake released, and
controlling the vehicle brake system so as to assume a second state in which the parking brake circuit is drained for applying the parking brake,
characterised by draining the service brake circuit in the second state for avoiding overload of the brake disc assembly.
Preferably, the service brake circuit is drained while the parking brake circuit is drained. Preferably, the step of draining the service brake circuit in the second state comprises draining the service brake circuit via the parking brake circuit. Preferably, a fluid flow is allowed from the service brake circuit to the parking brake circuit, and a fluid flow is prevented from the parking brake circuit to the service brake circuit. Preferably, where the vehicle brake system comprises a service brake control device adapted to selectively provide a fluid communication between the pressurising arrangement and the service brake via the service brake circuit, the step of controlling the vehicle brake system so as to assume the first state comprises providing the fluid communication between the parking brake and the pressurising arrangement independently of the service brake control device.
A computer program according to an aspect of the invention is provided comprising program code means for performing the steps of said method according to any embodiment herein. A computer readable medium carrying a computer program according to an aspect of the invention is provided comprising program code means for performing the steps of said method according to any embodiment herein when said program is run on a computer.
A controller for a vehicle brake system, the vehicle brake system according to an aspect of the invention is provided comprising a brake assembly providing a service brake and a parking brake having, a brake disc assembly in common, a service brake circuit connectable to the service brake, and a parking brake circuit connectable to the parking brake, the controller being configured to control the vehicle brake system so as to assume a first state in which a fluid communication is provided between the parking brake and a pressurising arrangement, for keeping the parking brake released, and
to control the vehicle brake system so as to assume a second state in which the parking brake circuit is drained for applying the parking brake,
wherein the controller is configured to control the vehicle brake system so that the service brake circuit is drained in the second state for avoiding overload of the brake disc assembly.
Preferably, the controller is configured to control the vehicle brake system so that the service brake circuit is drained in the second state, while the parking brake circuit is drained. Preferably, the controller is configured to control the vehicle brake system so that in the second state the service brake circuit is drained via the parking brake circuit.